With the development of the digital age and the advance of computer software and hardware technologies, users have increasingly higher requirements on the quality of digital images. In contrast to analog signals, digitization refers to processing information by digital means. For example, a digital camera (DC) is a camera that converts an optical image into electronic data by using an image sensor. The image sensor is a photosensitive charge-coupled device (CCD) or complementary metal-oxide-semiconductor (CMOS). During image acquisition, light passing through a lens is focused on a CCD or CMOS, the CCD or CMOS converts the light into an electrical signal and stores the electrical signal, then the electrical signal is converted into a digital signal by means of analog-to-digital conversion and stored, and an image can be formed after the digital signal is processed by a computer, a display, a printer, or the like.
After an image is digitized, resolution, dynamic range, signal-to-noise ratio and the like become important indicators for evaluating image quality. The image resolution refers to precision of an image, that is, the number of pixels per inch of the image, and the higher the resolution is, the more precise the image is and the more information can be displayed by an image of a same size. The dynamic range refers to a relative ratio between the brightest part and the darkest part of a scene, and is a technical term that describes mathematically the brightness level range of a given scene. The signal-to-noise ratio reflects image quality, that is, reflects whether an image is clean and free of noise. A large signal-to-noise ratio indicates a clean image picture without noticeable noise interference (manifested as “grain” and “snow”), which is pleasant to look at; a small signal-to-noise ratio indicates that there may be snow all over the picture, which seriously affects the image picture. There are many methods for improving image quality in the prior art, but usually one indicator can only be improved by sacrificing another indicator. For example, the dynamic range is improved by sacrificing the resolution, but as a result, image details are reduced; or the image resolution is improved by sacrificing the signal-to-noise ratio, but too much noise is caused.